More than 112 Tomahawk cruise missiles struck over 20 targets inside Libya today in the opening phase of an international military operation the Pentagon said was aimed at stopping attacks led by Libyan leader Moammar Gadhafi and enforcing a U.N.-backed no-fly zone.
"I want the American people to know that the use of force is not our first choice and it's not a choice I make lightly," Obama said. "But we cannot stand idly by when a tyrant tells his people that there will be no mercy."
The first air strikes, in what is being called Operation Odyssey Dawn, were launched from a mix of U.S. surface ships and one British submarine in the Mediterranean Sea at 2 p.m. ET, Vice Adm. William E. Gortney told reporters at a Pentagon briefing.
They targeted Libyan air defense missile sites, early warning radar and key communications facilities around Tripoli, Misratah, and Surt, but no areas east of that or near Benghazi. Because of darkness over Libya, Gortney said it was too early to determine the strikes' effectiveness.
Gortney said no U.S. troops were on the ground in Libya and that no U.S. aircraft participated in the initial attacks.
Libyan television reported that 48 people were killed and more than 150 wounded in the barrage, but there was no independent confirmation of the numbers.
Earlier today, as pro-Gadhafi forces battled towards the rebel stronghold of Benghazi, 20 French warplanes flew over the region in a show of force. And one jet fired on and destroyed an unidentified Libyan military vehicle, French Defense officials said.
At one point a fighter jet resembling a Libyan MiG 27 was shot down over the city, according to news reports from inside Libya.
Meanwhile, world leaders met in Paris to discuss the nature and scope of the international military intervention to make Gadhafi respect a U.N. Security Council resolution that authorized "all necessary measures" to protect Libyan civilians.
"We have every reason to fear that left unchecked, Gadhafi would commit unspeakable atrocities," Secretary of State Hillary Clinton told reporters following the meeting in Paris. "Further delay will only put more civilians at risk. So let me be very clear on the position of the United States: We will support an international coalition as it takes all necessary measures to enforce" the U.N. resolution.
But Pentagon officials cautioned that despite the initial military actions, an enforced no-fly zone over Libya was not yet in effect and will take time to establish.
"At this point we are creating the conditions to be able to set up a no fly zone, and once we have established and confirmed that the conditions are right then we will move forward into one of the next phases of the campaign," Gortney told reporters.
No U.S. aircraft will be involved in air strikes over Libya tonight, he said. "Our mission right now is to shape the battle space in such a way that our partners may take the lead in…execution."
As the campaign evolves, officials said, U.S. support aircraft would provide airborne surveillance, refueling and radar-jamming capabilities, and several F-16s may participate in patrols over no-fly zones above Tripoli and Benghazi.
In an audio statement broadcast on Libyan state TV, Gadhafi called the attacks a "crusade" against the Libyan people and called on Arab countries and African allies to come to his government's aid.
"We ask others to stand by us," he said, according to a translation of his remarks heard on Al Jazeera. "We must now open the weapons depot and arms to all Libyans."
Gadhafi warned the international coalition Friday not to interfere in Libyan affairs, calling the U.N. resolution "invalid" and appealing directly to world leaders, including President Obama, in a letter.
"Libya is not yours. Libya is for the Libyans," he said in the letter. "If you had found them taking over American cities with armed force, tell me what you would do."
Military action in Libya follows weeks of intensive, international diplomatic pressure on Gadhafi to cease the violence and pull back from rebel-held cities.
The Security Council approved a resolution late Thursday authorizing the international community to take "all necessary measures," short of sending in ground troops, to protect civilians in Libya, and to impose a no-fly zone. The resolution does not authorize taking out Gadhafi or regime change.
"The [U.N.] resolution that passed lays out very clear conditions that must be met," Obama said Friday.
"These terms are not subject to negotiation. If Gadhafi does not comply with the resolution, the international community will impose consequences and the resolution will be enforced through military action," he said.
Exactly what role the U.S. military would play in enforcement of the resolution remains unclear.
"We will support an international coalition as it takes all necessary measures to enforce the terms of resolution 1973," Secretary of State Clinton said today in Paris.
But Clinton declined to detail U.S. responsibilities in a supporting an attack, other than to say that the United States would offer "unique capabilities." She emphasized that the United States will not deploy ground troops in Libya.
During a meeting with a bipartisan group of members of Congress Friday, Obama said he expects active U.S. involvement in any military action would last just "days, not weeks," sources told ABC News.
Sources told ABC News that Obama's decision to support the use of force came Tuesday, following several days of internal administration deliberations and the realization that diplomatic efforts to stop the brutality of Gadhafi's regime weren't working.
Presented with intelligence about the push of the Gadhafi regime to the rebel stronghold of Benghazi, the president told his national security team, "What we're doing isn't stopping him."
Some in his administration, such as Clinton, had been pushing for stronger action, but it wasn't until Tuesday, administration sources tell ABC News, that the president became convinced sanctions and the threat of a no-fly zone wouldn't be enough.
"We are not going to use force to go beyond a well-defined goal, specifically the protection of civilians in Libya," Obama said Friday.
While the United States has been leading the charge behind the scenes, officials say, the administration deferred public action to the State Department and the United Nations in an effort to emphasize that the mission reflects a broad, international coalition, including support from Arab allies.
Gadhafi's son, Saif, told ABC News via a phone interview that the U.N. resolution is a "big mistake" and that if the United States wants to help, they should in fact help the government.
"We want to live in peace, so we want even Americans to help us get rid of the remnants of those people and to have a peaceful country, more democratic," he said. "If you want to help us, help us to, you know, to be democracy, more freedom, peaceful, not to threaten us with air strikes. We will not be afraid. Come on!"
U.S. Air Force Chief of Staff Gen. Norman Schwartz said Thursday it could take upwards of a week to fully establish a no-fly zone and that public comments by some that it could be done in a few days are "overly optimistic."
He acknowledged there are limited Air Force assets because most of them are in Iraq and Afghanistan, especially transport aircraft.
Last week, Department of National Intelligence director James Clapper said the Libyan air force was large in raw numbers, but only a small number of aircraft were actually flying.
A Pentagon analysis of Libya's air capabilities shows the overall readiness of Libyan aircraft is poor by Western standards and most aircraft are now dated or obsolete in terms of avionics or upgrades. Eighty percent of the air force is judged to be "non-operational and "overhaul and combat repair capability is also limited."
ABC News' Martha Raddatz, Huma Khan, Jake Tapper, Kirit Radia, and Luis Martinez contributed to this report.